Rip rap armours a slope against erosion, and sizing it has a twist the flat-ground calculators miss: the stone covers the sloped face, which is longer than the ground it sits on. Get that slope factor right, pick a layer thickness, and the tonnage follows. Here is the method.
The method
Work out the sloped area (bank length × slope length), multiply by the layer thickness for the volume, then by the placed density for tonnage. A 33 ft long, 13 ft run 2:1 slope (10 m × 4 m) is about 480 sq ft (45 m²) of face; at a 12 in (300 mm) layer that is roughly 18 yd³ (14 m³), or about 25 US tonswith a 10% voids allowance.
Why the slope factor matters
| Slope | Extra face area vs footprint |
|---|---|
| 2:1 | ~12% more |
| 1.5:1 | ~20% more |
| 1:1 | over 40% more |
Enter the run and slope ratio and the calculator scales the area up automatically, so you don’t come up short.
Rip Rap Calculator
Enter the slope size and rock layer for the tons and cubic yards, with the slope factor applied.
How thick, and what size stone?
A common rule is a layer 1.5–2× the median stone size (D50): about 12 in (300 mm) for 6 in stone, 18 in for 9 in stone. Bigger, faster water needs bigger stone — the Isbash equation sizes D50 from the flow velocity. For a designed job the D50 and thickness come from the drawing; switch the calculator to “size from flow” for a first-pass estimate on a straight, uniform bank.
Don’t skip the filter fabric
Almost always, a geotextile filter fabric (or a graded bedding-stone layer) goes under the riprap so the soil beneath can’t wash out through the gaps between stones — which would undermine the whole armour. Key the bottom row into a toe trench so the layer can’t slide down the slope.